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15.1 Have You Found a Bug?

If you are not sure whether you have found a bug, here are some guidelines:

  • If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash.
  • If the compiler produces invalid assembly code, for any input whatever (except an asm statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run.
  • If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug.

    However, you must double-check to make sure, because you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.

    For example, in many nonoptimizing compilers, you can write ‘x;’ at the end of a function instead of ‘return x;’, with the same results. But the value of the function is undefined if return is omitted; it is not a bug when GCC produces different results.

    Problems often result from expressions with two increment operators, as in f (*p++, *p++). Your previous compiler might have interpreted that expression the way you intended; GCC might interpret it another way. Neither compiler is wrong. The bug is in your code.

    After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.

  • If the compiler produces an error message for valid input, that is a compiler bug.
  • If the compiler does not produce an error message for invalid input, that is a compiler bug. However, you should note that your idea of “invalid input” might be someone else’s idea of “an extension” or “support for traditional practice”.
  • If you are an experienced user of one of the languages GCC supports, your suggestions for improvement of GCC are welcome in any case.

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